Personnel bucket brake for hydraulic cranes

ABSTRACT

A fluid-pressure responsive brake and actuation system therefor is disclosed which is operable to selectively brake the movement of a pivotally mounted, gravity leveled personnel bucket positioned atop an extensible boom of a crane or aerial device. The brake comprises a closed housing mounted on the bucket with a reciprocable rod centrally disposed within the housing and extending through the bucket wall into registry with an arcuate slot provided in an adjacent, independent braking plate fixably secured to the bucket supporting, shiftable section of the boom. A piston is journaled on the rod within the housing and a brake pad is externally attached to the rod at a point proximal to the braking plate. A high-bias, low-deflection spring within the housing on one side of the piston serves to bias the pad connected thereto into a normal frictional locking position with the braking plate. The opposed face of the piston communicates with a fluid-pressure chamber which in turn is joined by duct means to a two-way pressure-responsive shuttle valve interposed within the overall hydraulic system for raising and lowering the boom. In this manner the biasing spring acts to preclude pivotal movement of the bucket when the boom is stationary and the hydraulic system is at relatively low pressure. However, when the boom pivoting controls are actuated to move the boom, increased pressure against the shuttle valve opens the latter to direct pressurized motive fluid to the brake chamber to thereby move the rod in opposition to the spring. This serves to release the pad from braking contact with the adjacent plate, thus allowing the bucket to pivot under the influence of gravity to maintain the latter in a level position during raising or lowering of the boom.

United States Patent Reimbold, Jr. et al.

[ 1 Aug. 20, 1974 PERSONNEL BUCKET BRAKE FOR HYDRAULIC CRANES Inventors:James J. Reimbold, Jr., Overland Park, Kans.; Willard C. Kamberg,Pleasant Hill, Mo.

A. B. Chance Company, Centralia,

Filed: Aug. 22, 1973 Appl. No.: 390,611

Assignee:

[56] References Cited UNITED STATES PATENTS 3/1945 Brown 188/170 X11/1969 Eitel et al. 182/2 X 3,625,373 12/1971 Hull 188/170 X 3,653,4614/1972 Huxley et al 182/2 FOREIGN PATENTS OR APPLICATIONS 746,283 3/1956Great Britain 182/2 Primary Examiner-Duane A. Reger Attorney, Agent, orF irmSchmidt, Johnson, l-lovey & Williams [57] ABSTRACT A fluid-pressureresponsive brake and actuation system therefor is disclosed which isoperable to selectively brake the movement of a pivotally mounted,gravity leveled personnel bucket positioned atop an extensible boom of acrane or aerial device. The brake comprises a closed housing mounted onthe bucket with a reciprocable rod centrally disposed within the housingand extending through the bucket wall into registry with an arcuate slotprovided in an adjacent, independent braking plate fixably secured tothe bucket supporting, shiftable section of the boom. A piston isjournaled on the rod within the housing and a brake pad is externallyattached to the rod at a point proximal to the braking plate. Ahigh-bias, lowdeflection spring within the housing on one side of thepiston serves to bias the pad connected thereto into a normal frictionallocking position with the braking plate' The opposed face of the pistoncommunicates with a fluid-pressure chamber which in turn is joined byduct means to a two-way pressure-responsive shuttle valve interposedwithin the overall hydraulic system for raising and lowering the boom.In this manner the biasing spring acts to preclude pivotal movement ofthe bucket when the boom is stationary and the hydraulic system is atrelatively low pressure. However, when the boom pivoting controls areactuated to move the boom, increased pressure against the shuttle valveopens the latter to direct pressurized motive fluid to the brake chamberto thereby move the rod in opposition to the spring. This serves torelease the pad from braking contact with the adjacent plate, thusallowing the bucket to pivot under the influence of gravity to maintainthe latter in a level position during raising or lowering of the boom.

26 Claims, 5 Drawing Figures 25 20 s9 57 as PERSONNEL BUCKET BRAKE FORHYDRAULIC CRANES BACKGROUND This invention relates to a brake andactuation system therefor for use in selectively stopping movement of agravity leveled personnel bucket pivotally mounted on a swingable,hydraulically operated, extensible aerial boom. More particularly, it isconcerned with such a brake and system which serves to positively lockthe bucket when the boom is stationary, and which automatically releasesthe bucket during pivotal movement of the boom to permit a safe,continual leveling of the bucket during such movement.

Electrical utilities, contractors and others have for a number of yearsemployed mobile boom carrying units for construction and repairoperations in the field. These units normally comprise a vehicle such asa truck with a hydraulically operated, extensible aerial boom swingablymounted thereon. The vehicular devices often have at least one gravityleveled lightweight personnel bucket fabricated from insulativesynthetic resin material which is pivotally mounted at the outer end ofthe boom and is of a size to permit a workman to stand therein andperform needed construction or repair work on overhead areas, as forexample electrical lines or the upper extremities of supportingstructures such as poles. In practice, the worker enters the bucket atground level with the boom near a horizontal position and the boom isthen swung upwardly to a position convenient for the needed work. Uponcompletion thereof,

the boom is lowered to the ground to facilitate egress from the bucket.

As can be appreciated, it is very important that the pivotally mountedbucket be securely locked against pivotal motion when the boom is in anupright, extended, stationary work position. If the bucket were allowedto swing freely at these times, any movement of the lineman thereinwould induce unrestrained pivotal motions which could cause seriousmishaps. Furthermore, during raising or lowering of the boom it isequally as necessary to permit the bucket to pivot independently of theboom to achieve gravity induced leveling. If the bucket were locked withrespect to the boom during such operations, raising or lowering of thelatter could cause the bucket tovbecome angularly disposed with respectto the vertical. in extreme situations, the bucket could become orientedinsuch a manner that it would be difficult for the worker to preventhimself from falling from the bucket.

In order to overcome the problems mentioned above, conventionalpersonnel buckets have been provided with manually operable brakes.Thus, during swinging movement of the boom, the brake is in its open,unlocked position to allow the bucket to level itself under theinfluence of gravity. When the desired work position is reached and theboom becomes stationary, the worker standing in the bucket turns thebrake to its operative position to achieve the necessary steadying ofthe bucket. In a similar fashion, when it was desired to lower the boom,the worker moved the brake to its normal unlocked position to againpermit the bucket to continually level itself by swinging about thepivotal connection with the boom.

In practice however, such manually operable brakes have proven deficientin'several respects. Most importantly, these brakes are generally unsafebecause there is no quick and simple procedure for unlocking them fromground level. That is, if the worker within the bucket becomes injuredor unconscious due to electrical shock or other trauma, there is no wayto release the bucket brake to permit a safe, leveled lowering of thebucket. Hence, in emergency situations where speed is essential, themanually operated bucket brakes of the prior art have proven to bedetrimental, because of the necessity of carefully lowering the boom inorder that the worker does not fall from the bucket and further compoundhis injuries.

Therefore, there is a need in the art for a personnel bucket brake andactuation system therefor which is operable to normally lock the bucketwhen the supporting boom is in an extended, upright, stationary workposition, but which is capable of automatically releasing the bucketto.allow gravity-induced leveling movements thereof during raising orlowering of the boom.

SUMMARY Accordingly, it has been found that the above outlineddeficiencies can be overcome by providing a brake for personnel bucketswhich is normally mechanically biased to a locking position but which iskeyed to the overall hydraulic system of the boom to achieve a selectiveunlocking of the brake during swinging movement of the boom to effectraising and lowering thereof.

In gerenal, the brake comprises a closed housing mounted on the interiorface of a wall of the bucket, with a reciprocable actuating rodpositioned within the housing and extending through the bucket wall andinto registry with a slot within an adjacent braking plate. The latteris fixedly secured to the boom and is independent of the movement of thebucket. A brake pad is attached to the rod in proximal relationship tothe braking plate'such that the pad is capable of selectively engagingthe plate in a frictional manner to yield the desired braking function.

A fluid pressure responsive piston is slidably mounted within the closedhousing and is securely journaled upon the rod for reciprocationtherewith, thus dividing the housing into two separate, closed chambers.A biasing spring is positioned in one such chamber and is adapted tocommunicate with the piston and urge the latter and the attached rodinto a normalbraking position, i.e., with the external pad connected tothe rod being in frictional contact with the braking plate. The oppositeface of the piston communicates with a sealed fluid pressure chamber andthe latter has a hydraulic conduit operatively connected thereto. Theconduit is, in turn, connected to a pressure responsive valvingmechanism interposed within the hydraulic circuit for pivoting the boomto key the action of the ner, the rod and connected pad are moved out ofbraking contact with the brake plate which permits unrestrained pivotalmovements of the bucket under the influence of gravity.

The brake-actuating system in general comprises a supply of motive fluidwith conduit means defining a boom-actuating circuit between the supplyand a boom-actuating cylinder. Pump means are interposed within thecircuit in order to build up the desired magnitudes of hydraulicpressure, and flow directing valve means is provided to direct hydraulicfluid from the pump to the desired portions of the circuit to effectselective movement of the boom.

In a preferred form, a two-way shiftable shuttle valve is interposedwithin the hydraulic circuit and is connected to both the boom raisingand boom lowering segments of the hydraulic circuit. In this manner, abuildup of fluid pressure during either mode of operation of the boomtopping cylinder serves to open the shuttle valve to thereby permitpressurization of the brake assembly. Thus, during raising and loweringof the boom, the brake remains in its open, unlocked posi tion, whileacting to prevent pivotal movements of the bucket when the latter is ina desired work or stored position.

In another preferred embodiment, an internally piloted, one-way sequencevalve is interposed within each segment of the described hydrauliccircuit which is operable to permit flow of hydraulic fluid therethroughonly when the pressure thereof reaches a desired predeterminedmagnitude. This pressure limit is advantageously preset to a degreegreater than that required to release the bucket brake. In this way, thebrake is released an instant prior to actual movement of the boom, thusinsuring a smooth, jerk-free, continuous leveling of the bucket.

DRAWINGS line 4-4 of FIG. 2; and

FIG. 5 is a schematic representation of that portion of the hydrauliccircuit of the vehicle for raising and lowering the boom and including asecondary circuit interposed therein for the selective release of themechanically biased personnel bucket brake.

DETAILED DESCRIPTION The mobile crane shown in FIG. 1 and generallyreferred to by the numeral is especially adapted for use by utility orconstruction crews to lift heavy objects as well as lift a workman to anelevated height. It includes a conventional truck 12 stabilized by meansof rear outriggers 16 and provided with an extensible hydraulic boom 14mounted thereon. A lightweight personnel bucket 18 fabricated frominsulative synthetic resin material is pivotally mounted as at 20 on theouter, extensible endof boom 14.

Referring specifically to FIGS. 2-4, the details of the brake andassociated mechanisms are shown in detail. At the upper left of FIG. 2there is shown a boom tip plate 22 which forms an existing part of theshiftable outermost end of boom 14. An elongated axle member 24 issecured to plate 22 with the help of mounting structure 23 indisposition extending generally perpendicularly from plate 22 with apair of spaced, annular collars 26 and 28 journaled thereon at theopposed ends of the outer portion 25 thereof of reduced diameter.Depending from collar 26 is a generally semicircular braking plate 30having an arcuate slot 31 therein which underlies member 24 and iscoaxial therewith.

A metal hanger assembly 32 is pivotally suspended from portion 25 ofaxle member 24 between the spaced collars 26 and 28. The hanger includesa generally planar, triangularly-shaped back wall 34 with a pair offorwardly extending, inclined sidewalls 36 attached thereto. the hanger32 is provided with an axlereceiving assembly 37 at the apex thereofwhich includes a longitudinal bore 39 for the reception of the forwardportion 25 of axle 24. The forward locking collar 28 is secured to axleportion 25 by means of bolt 38, and serves to rotatably lock dependinghanger 32 onto axle member 24. At its lower end, housing 32 may bepositively connected to personnel bucket 18 by means of a series ofconventional bolts (not shown). In this manner, hanger 32 presents anupstanding extension of bucket 18 and provides a housing or niche forthe reception of the brake assembly generally referred to by the numeral40.

Brake assembly 40 includes a closed housing 42 mounted within the hanger32 in any convenient fashion, for example by welding baseplate 44 ontoback wall 34. A reciprocable actuating rod 46 centrally disposed withinhousing 42 extends through mated apertures 48 provided in baseplate 44and back wall 34 and into registry with the arcuate slot 31 in brakingplate 30. Rod 46 travels within the slot during all pivotal movements ofthe bucket, and therefore the slot is of complemental generallysemicircular configuration to the path of travel of the rod as is shownin FIG. 3.

A first braking pad 50 is attached to rod 46 at the end thereof remotefrom housing 42. Pad 50 can be of any desired shape and is preferablyfabricated from materials effective to give excellent frictionalengagement with depending brake plate 30. Pad 50 is mounted on rod 46 bymeans of annular bushing 52 and adjustable nut means 54, the latterserving as a spring pressure adjustment for the biasing spring withinhousing 42.

A second brake pad 56 is attached to back wall 34 and is positionedabout rod 46 between back wall 34 and plate 30 in slightly spacedrelationship from the latter. Both of the pads 50 and 56 are providedwith slipresistant brake lining elements presenting friction surfaces 58and 60 which directly contact opposed faces of braking plate 30sandwiched therebetween when the brake is in its normally lockedorientation.

A pressure-responsive piston 62 is positioned within housing 42 and ispositively connected to and joumaled on rod 46 for reciprocationtherewith. A high-bias, low deflection spring assembly 64 is operativelypositioned between the inner annular face of housing 42 and the lefthand surface of piston 62 for biasing the piston in a rightwarddirection viewing FIG. 2 to cause frictional braking contact betweenpads 50 and 56 and braking plate 30. In a preferred embodiment, thespring assembly 64 comprises a plurality of discrete, generallycupshaped, annular members fabricated from spring steel and slidablyjournaled on rod 46 with the open face of each member communicating withthe open face of another such member. In this manner, a spring isprovided which yields very high biasing forces when deflected onlyslightly. As can be appreciated, spring-biased pad 50 thus acts toslightly deflect plate and bring the latter into contact with stationarypad 56 which is slightly spaced from the plate. Therefore, both padscooperatively grip plate 30 in the stationary braking position, thusinsuring that the bucket is securely locked against movement.

Preferably a rigid, tubular stop sleeve 66 is placed in circumscribingrelationship to spring 64 which serves. to limit the possible travel ofpiston 62. Additionally, the chamber of housing 42 receiving springassembly 64 is vented as at 68 to preclude the possibility of pneumaticpressures building up therein during operation of the brake.

A fluid pressure chamber 70 is provided within housing 42 on the rightside thereof defined in part by the face of piston 62 remote from spring64. A hydraulic fitting 72 and associated conduit means 74 is attachedto housing 42 at a point allowing fluid communication with chamber 70.Leakage of hydraulic fluid from the latter is prevented by means ofannular seals 76 and 78 about rod 46 and piston 62 respectively.

It is apparent from FIG. 2 that when hydraulic fluid enters chamber 70under sufficient pressure to overcome the bias of spring 64, piston 62and rod 46 connected thereto are moved in a leftward direction as theassembly is depicted in FIG. 2. This serves to move pads and 56 out offrictional braking contact with plate 30, which consequently permitspivotal movement of the entire personnel bucket about the axis of axlemember 24. Hence, the operative position of brake assembly 40 isdependent upon the fluid pressure within chamber and as will be morefully explained below, this pressure is keyed to the fluid pressurewithin the overall boom raising and lowering hydraulic circuit such thatwhen the boom is stationary, the bucket is locked, and conversely whenthe boom is being raised or lowered, the bucket is free to pivotallymove under the influence of gravity.

In the embodiment shown in FIGS. 2-4, rod 46 extends through the end ofhousing 42 removed from wall 34. A manually actuatable eccentric cammingmember 78 is positioned externally of housing 42 in proximal relation tothe end of rod 46 extending out of the housing. Camming member 78 ismounted between a pair of spaced, generally triangularly-shaped braces80 and 82 extending rearwardly from housing 42. The member 78 is mountedupon transverse gudgeon 84 extending between braces 80 and 82 and isspring-loaded by means of biasing spring 86, which serves to retaincamming member 78 in the normal inoperative position depicted in FIG. 2.

The purpose of camming member 78 is to provide a means for manuallyreleasing brake assembly 40 in the event of a malfunction in thehydraulic actuating system therefor. That is, should a malfunction arisein the hydraulic circuit serving brake 42, the worker can turn cammingmember 78 by means of handle 88 tomechanically shift rod 46 leftwardlyto release the brake. To facilitate this operative effect, a detent 90is provided on the camming member at the point of its greatesteccentricity. The detent is dimensioned to seat on the end of rod 46 incommunication with the camming member to hold brake assembly 40 in itsreleased position.

Referring now to FIG. 5, there is shown a schematic representation ofthat portion of the boom-actuating hydraulic circuit which serves toraise and lower the boom. A secondary circuit is also shown which isoperable to direct motive fluid to the bucket brake assembly 40. Ingeneral, the system as illustrated includes a reservoir 92 of hydraulicfluid with conduit means defining a boom-actuating circuit 94 forpermitting selective raising and lowering of the boom by the doubleacting boom-actuating piston and cylinder assembly 96. The secondarybrake actuating circuit 95 is keyed to circuit 94 as explained below.

More specifically, a conduit 98 from reservoir 92 is connected to afixed displacement, gear-type pump 101 which is in turn communicateswith a four-way, three position directional flow valve 102. As shown inFIG. 5, the valve is in its neutral position wherein pressurizedhydraulic fluid is directed to bypass line 104 and back into reservoir92.

However, when valve 102 is raised to operatively interconnect portion106 thereof with line 98, hydraulic fluid is directed through conduit110 in order to effect an extension of the piston of boom-actuatingpiston and cylinder assembly 96. In this operational mode, fluid flowsthrough conduit 100 with a portion thereof being directed throughsecondary duct 108 which is connected to a two-way, shiftable shuttlevalve 110. The remainder of the pressurized fluid in conduit 100 thenflows to a conventional, internally piloted one-way sequence valveassembly 112.

The operation of valve assembly 112 is controlled by internal pilot line114 which is connected to the flowcontrol mechanism 115 thereof. Thelatter is opened to permit fluid flow when the pressure within line 114reaches a predetermined magnitude. Moreover, valve 112 is also operableto permit unrestricted fluid flow in the opposite direction, i.e., fromcylinder 96 to reservoir 92 through one-way check valve 113 forming apart of the overall assembly.

An externally piloted holding valve 116 is interposed within the boomraising portion of circuit 94 between valve assembly 112 and cylinder96. The operation of this valve does not interfere with boom raising andin controlled by means of external pilot line 118 interconnecting theflow control mechanism thereof and the boom lowering portion of circuit94 for purposes which are made clear hereinafter.

When pressurized hydraulic fluid from reservoir 92 passes through line100 and valves 112 and 116, it is then directed to the sealed extensionchamber 121 of cylinder 96. This serves to move the hydraulic piston 122and connecting rod 123 rightwardly as shown in FIG. 5, to thereby raisethe boom as desired. Additionally, the buildup of pressure within theextension leg of circuit 94 serves to actuate shuttle valve 110 topermit flow of pressurized motive fluid to brake assembly 40.

In one preferred embodiment, valve assembly 112 is initially set to openat a pressure of 500 psi, while a pressure of only psi is required toopen shuttle valve 110 and release brake 40. (It is to be understoodhowever, that these initial settings may be modified somewhat duringprimary testing of the system in order to optimize the operationthereof.) Therefore, during the buildup of pressure within line 100,hydraulic fluid under pressure is directed to brake assembly 40 torelease the personnel bucket before the boom begins to move. As can beappreciated, this allows the bucket to pivot in an unrestrained manneran instant before boom raising to thereby insure a smooth, jerk-free,gravity-induced leveling of the bucket.

In order to complete the hydraulic circuit 94, fluid within sealedretraction chamber 125 of cylinder 96 flows therefrom through line 124and valve assembly 126 and ultimately back to reservoir 92. In thismanner a smooth operation of the boom and associated bucket brakeassembly is achieved during all modes of operation.

In a similar fashion, if valve 102 is moved such that portion 128thereof is in operative communication with line 98, the boom is causedto lower. Motive fluid is first directed through line 124 towardsequence valve 126, with a portion thereof going through a secondbrake-actuating duct 130. By virture of the fact that the boom loweringoperation is identical in all respects to that of raising thereof, acomplete discussion is superfluous and therefore has been omitted. Ingeneral, however, a sequence valve assembly 126 is employed which isidentical to valve assembly 112 in order to insure uniformity in theresponse of brake assembly 40. Additionally, during lowering of the boomhydraulic fluid flows from chamber 121 of cylinder 92 through line 100to reservoir 92. Thus, during either operational mode, the unusedsegment of the circuit serves as a hydraulic fluid return system.

The purpose of holding valve 116 is to prevent the boom from creepingwhen in a raised position and to permit the boom extension leg ofcircuit 94 to carry return fluid to reservoir 92. Specifically, duringlowering of the boom, fluid is directed through pilot 118 to actuatevalve ll6 as shown in FIG. this permits return flow of fluid fromcylinder 96 back to reservoir 92 as described. Moreover, when the boomis in a static, raised position, valve 116 prevents return fluid flowtherefrom to preclude the possibility of boom creepage, or slowintermittent lowering. This same safety function also obtains ifpressure is lost within the system due to line breakage or the likesince valve 116 operates to prevent undesirable sudden, as well asgradual, boom depressions.

Once shuttle valve 110 opens during either boom raising or lowering,hydraulic fluid enters looped line 132 and pressurizes the latter whichis contained within the hollow, extensible portions of aerial boom 14.Brake assembly 40 is in turn pressurized, and the buildup of fluidpressure therein causes the brake to release as fully described above.In the preferred construction it has been found that the fluid pressurefor brake actuation should be below that required to actuate valve 110.This causes smooth release of brake assembly 40 prior to boom movementto insure smooth, jerk-free gravity-induced pivotal movement of bucket18.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

ally mounted on a shiftable boom, said brake assembly 6 acting to freethe bucket for gravitational leveling during raising and lowering of theboom and locking the bucket when said boom is stationary, thecombination with said bucket and boom of:

support structure secured to the bucket for movement therewith;

a brake plate fixedly carried by the boom in proximal relationship tosaid support structure; and

a hydraulically operated unit carried by the support structure andprovided with a shiftable brake pad located to move into frictionalengagement with the plate for locking the structure and thereby saidbucket to the boom, said unit including means for normally forcing thepad into frictional locking engagement with said plate and hydraulicallyactuated maans for shifting the pad out of said locking positionengaging the plate in response to raising or lowering of the boom.

2. The combination of claim 1 wherein is provided hydraulically actuatedpiston and cylinder means connected to the boom for raising and loweringthe latter, and hydraulic circuit means coupled to said piston andcylinder means for directing pressurized hydraulic fluid to either oneend or the other of said piston and cylinder means to effect raising orlowering of the boom, said hydraulically operated unit being operativelyconnected to said circuit for actuation by pressurized fluid directed toeither end of said piston and cylinder means to effect shifting of thebrake pad out of frictional engagement with said plate during eitherraising or lowering of the boom.

3. The combination of claim 2 wherein said unit includes a housing, ashiftable rod projecting from the housing toward said plate and carryingsaid pad thereon, a piston secured to the rod within the housing, meanswithin the housing engaging said piston on the side thereof forcing saidpad into frictional locking engagement with the plate, and means fordirecting pressurized hydraulic fluid from said circuit to the housingfor shifting the piston and thereby the rod through a displacement tomove the pad out of locking engagement with the plate against the actionof said biasing means during raising and lowering of the boom.

4. The combination of claim 3 wherein is provided means for effectingunlocking of the pad from said plate prior to raising or lowering of theboom by said piston and cylinder means.

5. A selectively actuatable, fluid-pressure responsive brake assemblyfor use with a personnel bucket pivotally mounted on a shiftable boom,said brake assembly acting to free the bcket for gravitational levelingduring raising and lowering of the boom and locking the bucket when saidboom is stationary, the combination with said bucket and boom of:

first and second plates positioned in adjacent, spaced relationship,said first plate .being fixedly connected to said boom, and said secondplate being connected to said bucket for movement therewith independentof said first plate;

a closed housing mounted on one of 'said plates;

a reciprocable actuating rod positioned within said housing andextending therefrom to pass through said adjacent plate, the latterbeing provided with a slot configured and arranged to allow said rod totravel therein during movement of said bucket;

a brake pad connected to said rod externally of said housing andpositioned to selectively communicate with said adjacent plate inresponse to reciprocation of said rod to effect a frictional, brakingcontact between the pad and adjacent plate;

a fluid pressure actuatable piston within said housing and journaled onsaid rod for reciprocation therewith;

biasing means within said housing on one side of said piston operable tobias the latter in a direction to cause braking contact between said padand adjacent plate, the opposite side of said piston communicating witha fluid pressure chamber within said housing; and

means operatively connected to said chamber for increasing the fluidpressure therein during raising and lowering of said boom, whereby, whensaid fluid pressure is of sufficient magnitude to overcome theoppositely directed force of said biasing means, said piston and rodmove in a direction to break the contact between said pad and adjacentplate, thereby freeing said bucket and allowing the latter to move.

6. The combination of claim 5, wherein said second plate comprises thebackwall of an upstanding hanger pivotally connected to said boom andattached to said bucket.

7. The combination of claim 6, wherein said housing is mounted on theface of said second plate removed from said first plate, saidreciprocable rod being centrally disposed therein and extending throughthe latter, said second plate being provided with an aperture therein topermit passage of said rod therethrough.

8. The combination of claim 3, wherein a first brake pad is attached tosaid rod and positioned to engage the face of said first plate removedfrom said second plate, a second apertured brake pad being attached tothe second plate and positioned in the space between the back wall andfirst plate with said rod extending therethrough, said first and secondpads normally cooperatively engaging opposite faces of said first plateto prevent movement of the bucket when said boom is stationary.

9. The combination of claim 8, wherein said housing is generallycylindrical in shape, said biasing means being positioned between saidpiston and second plate and operable to bias the piston in a directionaway fro the latter.

10. The combination of claim 9, wherein said biasing means is ahigh-bias, low-deflection spring unit.

11. The combination of claim 10, wherein said highbias, low-deflectionspring unit comprises a plurality of discrete, generally cup-shapedmembers fabricated of spring steel and slidably journaled on said rodwith the open face of each member facing the open face of an adjacentmember.

12. The combination of claim 11, wherein said highbias, low-deflectionspring unit is constructed to be deflected to an extent to release saidbrake by an oppos- 14. The combination of claim 13, wherein said manu-.ally actuatable means is a pivotally mounted, springbiased cammemberoperable to be turned about the pivot axis thereof and engageablewith said rod to effect axial movement thereof against the action ofsaid biasing means.

15. The combination of claim 14, wherein said cam member is providedwith a detent on the surface thereof operable to receive said rod andreleasably hold the latter in a brake-releasing position.

16. The combination of claim 5, wherein said housing is provided with anaperture in the portion thereof circumscribing said biasing means toeffect venting of said portion.

17. The combination of claim 5, wherein a stop sleeve is interposed inthe portion of said housing receiving said biasing means and disposed incircumscribing relationship to the latter for limiting the extent oftravel of said piston.

18. A braking system'for use with a movable personnel bucket brakemounted on a fluid-pressure actuated boom to permit free movement of thebucket during .raising and lowering of the boom and to lock the bucketwhen the boom is stationary, comprising:

a supply of motive fluid; actuating means for raising and lowering theboom;

conduit means operatively interconnecting said supply and the boomactuating means, and defining a hydraulic circuit therebetween to effectselective raising and lowering of said boom;

means interposed within said circuit operable to force motive fluidtherethrough under pressure;

directional means interposed within said circuit operable to selectivelydirect motive fluid through said circuit to a part of the boom actuatingmeans to effect raising of said boom, and for selectively directingmotive fluid through the circuit to another part of the boom actuatingmeans to effect lowering of the boom;

pressure responsive means interposed within said circuit operable topermit flow therethrough when the fluid pressure within the circuitreaches a first predetermined magnitude;

selectively actuatable fluid pressure responsive brake means adapted tobe operatively connected to said personnel bucket and operable to holdthe latter against movement when said boom is stationary, said brakemeans being selectively actuatable to release said bucket and allowmovement thereof when the fluid pressure therein reaches a secondpredetermined magnitude; and

conduit means interconnecting said pressure respon sive means and brakemeans whereby, when the fluid pressure within the circuit reaches saidfirst predetermined magnitude, motive fluid is directed to said brakemeans, said brake being actuated to release said bucket and allowmovement thereof as soon as fluid under a pressure equal to or exceedingsaid second predetermined magnitude thereof is directed to said brakemeans from the circuit. 19. The system of claim 18, said conduit meansincluding a first boom raising portion interconnecting said directionalmeans and boom to permit pressurized fluid-flow therethrough to raisethe boom, and a second boom lowering portion interconnecting saiddirectional means and boom to permit pressurized fluid-flow therethroughto lower the boom, said directional means being selectively positionableto direct pressurized fluid through said first and second portions asdesired.

20. The system of claim 19, wherein each of said conduit portions isconnected to said supply of motive fluid such that when pressurizedfluid is being directed through one of said portions, the other portionserves as a return conduit for returning fluid from said boom actuatingmeans to the supply.

21. The system of claim 20, wherein an internally piloted, one-waysequence valve is interposed within said respective first and secondportions between said boom actuating means and the directional means,said sequence valves being operable to perclude flow of motive fluidthrough said respective portions in a direction from the directionalmeans to the boom actuating means until the fluid pressure thereinreaches a third predetermined magnitude, and operable to allowunrestricted flow of motive fluid therethrough from the boom actuatingmeans to the supply.

22. The system of claim 21 wherein said pressure responsive meanscomprises:

a first duct connected to said first conduit portion on the input sideof the sequence valve interposed within the latter;

a second duct connected to said second conduit portion on the input sideof the sequence valve interposed within the latter; and

a two-way, shiftable shuttle valve interconnecting said first and secondduets with conduit means interconnecting the shuttle valve and brakemeans, said shuttle valve being adapted to permit flow of motive fluidto said brake means when the fluid pressure within either of said ductsreaches said first predetermined magnitude.

23. The system of claim 22, wherein said first predetermined magnitudeis about 130 psi, said second predetermined magnitude is less than 130psi, and said third predetermined magnitude is about 500 psi.

24. The system of claim 18, wherein said means for forcing motive fluidthrough said conduit means under pressure is a fixed displacement pump.

25. The system of claim 24, wherein said directional means is interposedin said circuit on the output side of said pump and comprises afour-way, three-position directional flow valve.

26. The system of claim 18, wherein brake means comprises:

first and second plates positioned in an adjacent,

spaced relationship, said first plate being adapted to be fixedlyconnected to said boom, and said second plate being adapted to beconnected to saidbucket for movement therewith independent of said firstplate;

a closed housing mounted on one of said plates;

a reciprocable actuating rod positioned within said housing andextending therefrom to pass through an adjacent plate, the latter beingprovided with a slot configured and arranged to allow said rod to traveltherein during movement of said bucket;

a brake pad connected to said rod externally of said housing andpositioned to selectively engage said adjacent plate in response toreciprocation of said rod to effect a frictional, braking contactbetween the pad and adjacent plate;

a fluid pressure actuatable piston within said housing and journaled onsaid rod for reciprocation therewith; and

biasing means within said housing on one side of said piston operable tobias the latter in a direction to cause braking contact between saidpadand adjacent plate, the opposite side of said piston communicatingwith a fluid pressure chamber within said housing.

1. A selectively actuatable, fluid-pressure responsive brake assemblyfor use with a personnel bucket pivotally mounted on a shiftable boom,said brake assembly acting to free the bucket for gravitational levelingduring raising and lowering of the boom and locking the bucket when saidboom is stationary, the combination with said bucket and boom of:support structure secured to the bucket for movement therewith; a brakeplate fixedly carried by the boom in proximal relationship to saidsupport structure; and a hydraulically operated unit carried by thesupport structure and provided with a shiftable brake pad located tomove into frictional engagement with the plate for locking the structureand thereby said bucket to the boom, said unit including means fornormally forcing the pad into frictional locking engagement with saidplate and hydraulically actuated maans for shifting the pad out of saidlocking position engaging the plate in response to raising or loweringof the boom.
 2. The combination of claim 1 wherein is providedhydraulically actuated piston and cylinder means connected to the boomfor raising and lowering the latter, and hydraulic circuit means coupledto said piston and cylinder means for directing pressurized hydraulicfluid to either one end or the other of said piston and cylinder meansto effect raising or lowering of the boom, said hydraulically operatedunit being operatively connected to said circuit for actuation bypressurized fluid directed to either end of said piston and cylindermeans to effect shifting of the brake pad out of frictional engagementwith said plate during either raising or lowering of the boom.
 3. Thecombination of claim 2 wherein said unit includes a housing, a shiftablerod projecting from the housing toward said plate and carrying said padthereon, a piston secured to the rod within the housing, means withinthe housing engaging said piston on the side thereof forcing said padinto frictional locking engagement with the plate, and means fordirecting pressurized hydraulic fluid from said circuit to the housingfor shifting the piston and thereby the rod through a displacement tomove the pad out of locking engagement with the plate against the actionof said biasing means during raising and lowering of the boom.
 4. Thecombination of claim 3 wherein is provided means for effecting unlockingof the pad from said plate prior to raising or lowering of the boom bysaid piston and cylinder means.
 5. A selectively actuatable,fluid-pressure responsive brake assembly for use with a personnel bucketpivotally mounted on a shiftable boom, said brake assembly acting tofree the bcket for gravitational leveling during raising and lowering ofthe boom and locking the bucket when said boom is stationary, thecombination with said bucket and boom of: first and second platespositioned in adjacent, spaced relationship, said first plate beingfixedly connected to said boom, and said second plate being connected tosaid bucket for movement therewith independent of said first plate; aclosed housing mounted on one of said plates; a reciprocable actuatingrod positioned within said housing and extending therefrom to passthrough said adjacent plate, the latter being provided with a slotconfigured and arranged to allow said rod to travel therein duringmovement of said bucket; a brake pad connected to said rod externally ofsaid housing and positioned to selectively communicate with saidadjacent plate in response to reciprocation of said rod to effect africtional, braking contact between the pad and adjacent plate; a fluidpressure actuatable piston within said housing and journaled on said rodfor reciprocation therewith; biasing means within said housing on oneside of said piston operable to bias the latter in a direction to causebraking contact between said pad and adjacent plate, the opposite sideof said piston communicating with a fluid pressure chamber within saidhousing; and means operatively connected to said chamber for increasingthe fluid pressure therein during raising and lowering of said boom,whereby, when said fluid pressure is of sufficient magnitude to overcomethe oppositely directed force of said biasing means, said piston and rodmove in a direction to break the contact between said pad and adjacentplate, thereby freeing said bucket and allowing the latter to move. 6.The combination of claim 5, wherein said second plate comprises thebackwall of an upstanding hanger pivotally connected to said boom andattached to said bucket.
 7. The combination of claim 6, wherein saidhousing is mounted on the face of said second plate removed from saidfirst plate, said reciprocable rod being centrally disposed therein andextending through the latter, said second plate being provided with anaperture therein to permit passage of said rod therethrough.
 8. Thecombination of claim 3, wherein a first brake pad is attached to saidrod and positioned to engage the face of said first plate removed fromsaid second plate, a second apertured brake pad being attached to thesecond plate and positioned in the space between the back wall and firstplate with said rod extending therethrough, said first and second padsnormally cooperatively engaging opposite faces of said first plate toprevent movement of the bucket when said boom is stationary.
 9. Thecombination of claim 8, wherein said housing is generally cylindrical inshape, said biasing means being positioned between said piston andsecond plate and operable to bias the piston in a direction away fromthe latter.
 10. The combination of claim 9, wherein said biasing meansis a high-bias, low-deflection spring unit.
 11. The combination of claim10, wherein said high-bias, low-deflection spring unit comprises aplurality of discrete, generally cup-shaped members fabricated of springsteel and slidably journaled on said rod with the open face of eachmember facing the open face of an adjacent member.
 12. The combinationof claim 11, wherein said high-bias, low-deflection spring unit isconstructed to be deflected to an extent to release said brake by anopposing pressure within said fluid pressure chamber of under 130 psi.13. The cOmbination of claim 9, wherein said rod extends through the endof said housing removed from said plates, said housing being providedwith manually actuatable means externally mounted on said end thereofoperable to act on said rod to move the latter in a direction towardsaid plates to release said brake.
 14. The combination of claim 13,wherein said manually actuatable means is a pivotally mounted,spring-biased cam member operable to be turned about the pivot axisthereof and engageable with said rod to effect axial movement thereofagainst the action of said biasing means.
 15. The combination of claim14, wherein said cam member is provided with a detent on the surfacethereof operable to receive said rod and releasably hold the latter in abrake-releasing position.
 16. The combination of claim 5, wherein saidhousing is provided with an aperture in the portion thereofcircumscribing said biasing means to effect venting of said portion. 17.The combination of claim 5, wherein a stop sleeve is interposed in theportion of said housing receiving said biasing means and disposed incircumscribing relationship to the latter for limiting the extent oftravel of said piston.
 18. A braking system for use with a movablepersonnel bucket brake mounted on a fluid-pressure actuated boom topermit free movement of the bucket during raising and lowering of theboom and to lock the bucket when the boom is stationary, comprising: asupply of motive fluid; actuating means for raising and lowering theboom; conduit means operatively interconnecting said supply and the boomactuating means, and defining a hydraulic circuit therebetween to effectselective raising and lowering of said boom; means interposed withinsaid circuit operable to force motive fluid therethrough under pressure;directional means interposed within said circuit operable to selectivelydirect motive fluid through said circuit to a part of the boom actuatingmeans to effect raising of said boom, and for selectively directingmotive fluid through the circuit to another part of the boom actuatingmeans to effect lowering of the boom; pressure responsive meansinterposed within said circuit operable to permit flow therethrough whenthe fluid pressure within the circuit reaches a first predeterminedmagnitude; selectively actuatable fluid pressure responsive brake meansadapted to be operatively connected to said personnel bucket andoperable to hold the latter against movement when said boom isstationary, said brake means being selectively actuatable to releasesaid bucket and allow movement thereof when the fluid pressure thereinreaches a second predetermined magnitude; and conduit meansinterconnecting said pressure responsive means and brake means whereby,when the fluid pressure within the circuit reaches said firstpredetermined magnitude, motive fluid is directed to said brake means,said brake being actuated to release said bucket and allow movementthereof as soon as fluid under a pressure equal to or exceeding saidsecond predetermined magnitude thereof is directed to said brake meansfrom the circuit.
 19. The system of claim 18, said conduit meansincluding a first boom raising portion interconnecting said directionalmeans and boom to permit pressurized fluid-flow therethrough to raisethe boom, and a second boom lowering portion interconnecting saiddirectional means and boom to permit pressurized fluid-flow therethroughto lower the boom, said directional means being selectively positionableto direct pressurized fluid through said first and second portions asdesired.
 20. The system of claim 19, wherein each of said conduitportions is connected to said supply of motive fluid such that whenpressurized fluid is being directed through one of said portions, theother portion serves as a return conduit for returning fluid from saidboom actuating means to the supply.
 21. The system of claim 20, whereinan internally piloted, one-way sequencE valve is interposed within saidrespective first and second portions between said boom actuating meansand the directional means, said sequence valves being operable toperclude flow of motive fluid through said respective portions in adirection from the directional means to the boom actuating means untilthe fluid pressure therein reaches a third predetermined magnitude, andoperable to allow unrestricted flow of motive fluid therethrough fromthe boom actuating means to the supply.
 22. The system of claim 21wherein said pressure responsive means comprises: a first duct connectedto said first conduit portion on the input side of the sequence valveinterposed within the latter; a second duct connected to said secondconduit portion on the input side of the sequence valve interposedwithin the latter; and a two-way, shiftable shuttle valveinterconnecting said first and second ducts with conduit meansinterconnecting the shuttle valve and brake means, said shuttle valvebeing adapted to permit flow of motive fluid to said brake means whenthe fluid pressure within either of said ducts reaches said firstpredetermined magnitude.
 23. The system of claim 22, wherein said firstpredetermined magnitude is about 130 psi, said second predeterminedmagnitude is less than 130 psi, and said third predetermined magnitudeis about 500 psi.
 24. The system of claim 18, wherein said means forforcing motive fluid through said conduit means under pressure is afixed displacement pump.
 25. The system of claim 24, wherein saiddirectional means is interposed in said circuit on the output side ofsaid pump and comprises a four-way, three-position directional flowvalve.
 26. The system of claim 18, wherein brake means comprises: firstand second plates positioned in an adjacent, spaced relationship, saidfirst plate being adapted to be fixedly connected to said boom, and saidsecond plate being adapted to be connected to said bucket for movementtherewith independent of said first plate; a closed housing mounted onone of said plates; a reciprocable actuating rod positioned within saidhousing and extending therefrom to pass through an adjacent plate, thelatter being provided with a slot configured and arranged to allow saidrod to travel therein during movement of said bucket; a brake padconnected to said rod externally of said housing and positioned toselectively engage said adjacent plate in response to reciprocation ofsaid rod to effect a frictional, braking contact between the pad andadjacent plate; a fluid pressure actuatable piston within said housingand journaled on said rod for reciprocation therewith; and biasing meanswithin said housing on one side of said piston operable to bias thelatter in a direction to cause braking contact between said pad andadjacent plate, the opposite side of said piston communicating with afluid pressure chamber within said housing.